hey choader
your light guy sounds like he's been around the block a few times
it's good to find someone who knows what's up...
as for the calcs, you can use any units you like. it doesn't matter - it's really the ratio of distances you're looking at, at least in terms of the inverse square law.
absolute brightness is another story, and it is very difficult to calculate. there are too many variables, mostly in the design and construction of the reflector. to be able to compare different lamps effectively, you need to do some emperical measurements - using the same reflectors for each. you have to remove the variables if you can't quantify them precisley. i think that's one of the little details the clouds up discussions about lighting, the numbers are pretty much useless without a solid baseline to relate them to.
anyway, in macro terms, you can think in terms of the distance/lumens relationship another way. say you have a light very close to your plants. because it's (for all practical purposes) a point source, it will illuminate a small area. now as you increase the distance, the coverage becomes greater (here's the reflector factor i was referring to), and dispersion comes into play.
so if you're growing a large bushy plant, you want to cover it as best you can - think about the large fan leaves off to the sides of the bush. so in that case, having a cool light close to the plant isn't doing any good...so you pull it back to get coverage, and your lu psf goes down the drain.
on the other hand, you could use a lot of those lamps - but that gets expensive real fast. always a trade-off...
that's also one of the reasons the sun is so effective...you've got 10k lu psf on the top of the plant, on the sides, on the bottom...of course whatever is exposed to the sun...the inverse square law doesn't apply.
now if it would just stop raining
